linux-kernel - Re: [PATCH v4 1/2] usbip: Skip DMA mapping and unmapping for urb at vhci

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Message-ID: <b5c6c9e5-0855-c055-6bb5-47dbc497f606@kernel.org>
Date:   Tue, 6 Aug 2019 09:38:54 -0600
From:   shuah <shuah@...nel.org>
To:     Suwan Kim <suwan.kim027@...il.com>
Cc:     valentina.manea.m@...il.com, gregkh@...uxfoundation.org,
        stern@...land.harvard.edu, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, shuah <shuah@...nel.org>
Subject: Re: [PATCH v4 1/2] usbip: Skip DMA mapping and unmapping for urb at
 vhci

On 8/6/19 9:32 AM, Suwan Kim wrote:
> On Tue, Aug 06, 2019 at 09:11:30AM -0600, shuah wrote:
>> On 8/6/19 6:31 AM, Suwan Kim wrote:
>>> vhci doesn’t do DMA for remote device. Actually, the real DMA
>>> operation is done by network card driver. vhci just passes virtual
>>> address of the buffer to the network stack, so vhci doesn’t use and
>>> need dma address of the buffer of the URB.
>>>
>>> But HCD provides DMA mapping and unmapping function by default.
>>> Moreover, it causes unnecessary DMA mapping and unmapping which
>>> will be done again at the NIC driver and it wastes CPU cycles.
>>> So, implement map_urb_for_dma and unmap_urb_for_dma function for
>>> vhci in order to skip the DMA mapping and unmapping procedure.
>>>
>>> When it comes to supporting SG for vhci, it is useful to use native
>>> SG list (urb->num_sgs) instead of mapped SG list because DMA mapping
>>> fnuction can adjust the number of SG list (urb->num_mapped_sgs).
>>> And vhci_map_urb_for_dma() prevents isoc pipe from using SG as
>>> hcd_map_urb_for_dma() does.
>>>
>>> Signed-off-by: Suwan Kim <suwan.kim027@...il.com>
>>> ---
>>> v3 - v4:
>>> - Replace WARN_ON() with pr_err() in the error path.
>>>
>>> v2 - v3
>>> - Move setting URB_DMA_MAP_SG flag to the patch 2.
>>> - Prevent isoc pipe from using SG buffer.
>>>
>>> v1 - v2
>>> - Add setting URB_DMA_MAP_SG flag in urb->transfer_flags to tell
>>> stub driver to use SG buffer.
>>> ---
>>>    drivers/usb/usbip/vhci_hcd.c | 19 +++++++++++++++++++
>>>    1 file changed, 19 insertions(+)
>>>
>>> diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
>>> index 000ab7225717..429e4e989f38 100644
>>> --- a/drivers/usb/usbip/vhci_hcd.c
>>> +++ b/drivers/usb/usbip/vhci_hcd.c
>>> @@ -1288,6 +1288,22 @@ static int vhci_free_streams(struct usb_hcd *hcd, struct usb_device *udev,
>>>    	return 0;
>>>    }
>>> +static int vhci_map_urb_for_dma(struct usb_hcd *hcd, struct urb *urb,
>>> +				gfp_t mem_flags)
>>> +{
>>> +	if (usb_endpoint_xfer_isoc(&urb->ep->desc) && urb->num_sgs) {
>>> +		pr_err("SG is not supported for isochronous transfer\n");
>>
>> Any reason to not use dev_err()?
> 
> Because some codes in vhci_hcd.c use pr_err().There is no other
> reason. However, dev_err() seems more appropriate than pr_err().
> I will replace pr_err() with dev_err(urb->dev->dev, "SG is ...")
> Is it ok?
> 

Please. This way we will have the device information. pr_err()
won't us that. In general I prefer dev_* if dev is available in
the code path, which is the case here.

thanks,
-- Shuah